Why generate POP_TOP after an "import from?"

[Alexandre Brault]

On 2020-04-17 2:22 p.m., Adam Preble wrote:
> Given this in Python 3.6.8:
>
> from dis import dis
>
> def import_from_test():
>     from sys import path
>
>>>> dis(import_from_test)
>    2           0 LOAD_CONST               1 (0)
>                2 LOAD_CONST               2 (('path',))
>                4 IMPORT_NAME              0 (sys)
>                6 IMPORT_FROM              1 (path)
>                8 STORE_FAST               0 (path)
>               10 POP_TOP
>               12 LOAD_CONST               0 (None)
>               14 RETURN_VALUE
>
> I don't understand why there's a POP_TOP there that I don't get for an import_name grammatical statement.
>
> IMPORT_NAME needs to eat the top two entries of the stack for level and the from-list. BTW I don't know what level is for either since my science projects have always had it be zero, but that's another question.
>
> IMPORT_NAME will the push the module on to the stack.
>
> IMPORT_FROM will import path from the module on the stack, and push that result on the stack.
>
> STORE_FAST will store path for use, finally "modifying the namespace."
>
> At this point, my conceptual stack is empty. If I POP_TOP then I have nothing to pop and the world would end. Yet, it doesn't. What am I missing?

You can get an idea of what you're missing if you import multiple names 
from a module at once:

 >>> def f():
...     from sys import path, argv
...
 >>> dis.dis(f)
   2           0 LOAD_CONST               1 (0)
               2 LOAD_CONST               2 (('path', 'argv'))
               4 IMPORT_NAME              0 (sys)
               6 IMPORT_FROM              1 (path)
               8 STORE_FAST               0 (path)
              10 IMPORT_FROM              2 (argv)
              12 STORE_FAST               1 (argv)
              14 POP_TOP
              16 LOAD_CONST               0 (None)
              18 RETURN_VALUE

As shown here (and confirmed by the doc of the IMPORT_FROM opcode), 
IMPORT_FROM loads an attribute from the module on top of the stack, but 
doesn't pop the module. The POP_TOP instruction is what does.